Figures / Tables:

Figure 1: Thermodynamic stability and synthesis of mixed Mn2+/V4+ disordered rocksalt oxyfluorides. a. Computed phase diagram of the MnO/Li2VO3/LiF alloy space. The temperature–marked contours denote the extent of the binodal, starting from the MnO and Li2VO3 endpoints, at the given temperature. The color–coded overlay plots the theoretical gravimetric capacitymore » of compositions across the phase diagram based on Li and transition metal redox content. The percolation threshold marks compositions exceeding the 10% Li-excess requirement.2 The inset illustrates the region of the phase diagram we focus on, with the stoichiometric, transition metal-rich, and Li-rich compositions to synthesize. b. XRD profiles and refined lattice parameters of the synthesized disordered rocksalt compounds.« less

Mn-based Li-excess cation-disordered rocksalt (DRX) oxyfluorides are promising candidates for next-generation rechargeable battery cathodes owing to their large energy densities, the earth abundance, and low cost of Mn. In this work, we synthesized and electrochemically tested four representative compositions in the Li-Mn-O-F DRX chemical space with various Li and F content. Although all compositions achieve higher than 200 mAh g -1 initial capacity and good cyclability, we show that the Li-site distribution plays a more important role than the metal-redox capacity in determining the initial capacity, whereas the metal-redox capacity is more closely related to the cyclability of the materials.more » We apply these insights and generate a capacity map of the Li-Mn-O-F chemical space, LixMn 2-xO 2-yF y (1.167 ≤ x ≤ 1.333, 0 ≤ y ≤ 0.667), which predicts both accessible Li capacity and Mn-redox capacity. This map provides the design of compounds that balance high capacity with good cyclability.« less

Significant research effort has focused on improving the specific energy of lithium-ion batteries for emerging applications, such as electric vehicles. Recently, a rock salt–type Li 4Mn 2O 5 cathode material with a large discharge capacity (~350 mA·hour g –1) was discovered. However, a full structural model of Li 4Mn 2O 5 and its corresponding phase transformations, as well as the atomistic origins of the high capacity, warrants further investigation. We use first-principles density functional theory (DFT) calculations to investigate both the disordered rock salt–type Li 4Mn 2O 5 structure and the ordered ground-state structure. The ionic ordering in the ground-statemore » structure is determined via a DFT-based enumeration method. We use both the ordered and disordered structures to interrogate the delithiation process and find that it occurs via a three-step reaction pathway involving the complex interplay of cation and anion redox reactions: (i) an initial metal oxidation, Mn 3+→Mn 4+ (Li xMn 2O 5, 4 > x > 2); (ii) followed by anion oxidation, O 2– → O 1– (2 > x > 1); and (iii) finally, further metal oxidation, Mn 4+ → Mn 5+ (1 > x > 0). This final step is concomitant with the Mn migration from the original octahedral site to the adjacent tetrahedral site, introducing a kinetic barrier to reversible charge/discharge cycles. Armed with this knowledge of the charging process, we use high-throughput DFT calculations to study metal mixing in this compound, screening potential new materials for stability and kinetic reversibility. We predict that mixing with M = V and Cr in Li 4(Mn,M) 2O 5 will produce new stable compounds with substantially improved electrochemical properties.« less

Advanced lithium-ion batteries for renewable energy storage applications have become a major research interest in recent years. Much better performance can be realized by improvements in the material surface design, especially for the cathode materials. Here, we present a new design for a surface protective layer formed via a facile aqueous solution process in which a nano-architectured layer of LiF/FeF 3 is epitaxially grown on bulk hierarchical Li-rich cathode Li[Li 0.2Ni 0.2Mn 0.6]O 2. Coin cell tests of this material in the voltage range of 2–4.8 V indicated a high reversible capacity (260.1 mA h g -1 at 0.1 C),more » superior rate performance (129.9 mA h g -1 at 20 C), and excellent capacity retention. Differential scanning calorimetry showed good thermal stability. The enhanced capacity and cycling stability are attributed to the suppression of interfacial side reactions as well as the conversion reaction resulting from the introduction of LiF/FeF 3 as a surface protective layer.« less

We report the recent discovery of Li-excess cation-disordered rock salt cathodes has greatly enlarged the design space of Li-ion cathode materials. Evidence of facile lattice fluorine substitution for oxygen has further provided an important strategy to enhance the cycling performance of this class of materials. Here, a group of Mn 3+–Nb 5+-based cation-disordered oxyfluorides, Li 1.2Mn 3+0.6+0.5xNb 5+0.2-0.5xO 2-xF x (x = 0, 0.05, 0.1, 0.15, 0.2) is investigated and it is found that fluorination improves capacity retention in a very significant way. Combining spectroscopic methods and ab initio calculations, it is demonstrated that the increased transition-metal redoxmore » (Mn 3+/Mn 4+) capacity that can be accommodated upon fluorination reduces reliance on oxygen redox and leads to less oxygen loss, as evidenced by differential electrochemical mass spectroscopy measurements. Furthermore, it is found that fluorine substitution also decreases the Mn 3+-induced Jahn–Teller distortion, leading to an orbital rearrangement that further increases the contribution of Mn-redox capacity to the overall capacity.« less

Lithium-ion batteries (LIBs) have been used widely in portable electronics, and hybrid-electric and all-electric vehicles for many years. However, there is a growing need to develop new cathode materials that will provide higher cell energy densities for advanced applications. Several candidates, including Li 2MnO 3-stabilized LiM'O 2 (M' = Mn/Ni/Co) structures, Li 2Ru 0.75Sn 0.25O 3 (i.e., 3Li 2RuO 3–Li 2SnO 3), and disordered Li 2MoO 3–LiCrO 2 compounds can yield capacities exceeding 200 mA h g -1, alluding to the constructive role that Li 2MO 3 (M 4+) end-member compounds play in the electrochemistry of these systems. Here inmore » this paper, we catalog the family of Li 2MO 3 compounds as active cathodes or inactive stabilizing agents using high-throughput density functional theory (HT-DFT). With an exhaustive search based on design rules that include phase stability, cell potential, resistance to oxygen evolution, and metal migration, we predict a number of new Li 2M IO 3–Li 2M IIO 3 active/inactive electrode pairs, in which MI and MII are transition- or post-transition metal ions, that can be tested experimentally for high-energy-density LIBs.« less